1. Field of the Invention
This invention is directed to circulating fluidized bed reactors and more particularly, to the introduction of combustion gases derived from liquid and gaseous fuels into such reactors.
2. Description of Related Art
A fluid bed reactor is a vertical, cylindrical vessel which generally is comprised of three functional sections: (a) the windbox, or lower section, located beneath a constriction plate, (b) the fluid bed (bubbling regime) or dense phase (CFB regime), immediately above the constriction plate, and (c) the freeboard (bubbling regime) or dilute phase (CFB regime), the uppermost section where solids are disengaged from the upward moving gas stream.
Fluidization is the suspension of solids by an upward gas stream. The suspended mixture of solids with gases resembles a fluid. A fluid bed is a mixture of solids and gases in suspension contained in the lower-middle portion of the fluid bed reactor. The fluid bed is bound by the reactor walls on the sides and contained by the constriction plate on the bottom. The top of the bed looks like an irregular splashing, boiling surface. A reactor that has such a fluid bed is known in the art as a "bubbling" fluid bed reactor. A reactor in which particles are recirculated is known as a circulating fluidized bed (CFB) reactor. The boundary between the upper and lower zones becomes much less distinct. Pulsations or "bubbling" is diminished in a CFB reactor. Typically CFB reactors are utilized in the boiler industry wherein solid fuels, i.e., coal, coke, wood, rubber, etc., are fired in the reactor.
Fluidized bed reactors are extremely versatile apparatuses, which, in various forms, can carry out the processes of drying, sizing, roasting, calcining, heat treatment of solids with gases in the chemical, metallurgical and other materials processing fields, and the generation of hot gases, including steam, for use in driving electric power generation equipment. Fluidized bed reactors have also been successfully applied to the incineration of combustible waste streams such as sewage sludge and oil refinery wastes.
Direct injection of liquids, solids and gaseous fuels into "bubbling" fluid bed reactors is known in the art. Since "bubbling" fluid bed reactors operate at relatively low velocities, a high percentage of the heat released (oxidation) usually occurs in the fluid bed rather than in the freeboard zone, the cyclone, or the exhaust gas ducts. Excessive heat release in the freeboard zone causes severe operational problems which include, but are not limited to, scaling, poor temperature control, increased energy requirements, poor product quality and overheating of the cyclone and exhaust gas ducts.
CFB reactors are operated at higher velocities. Direct injection of solid fuels into CFB reactors produces heat release primarily in the lower zone (dense phase). Contrary to the "bubbling" lower fluid bed reactors, some heat release (oxidation) in the upper zone (dilute phase) does not pose a significant problem. However, it has been found that direct injection of liquid and gaseous fuels into conventional CFB reactors greatly reduces the heat release (oxidation) in the lower zone (dense phase) thereby causing overheating of the upper zone (dilute phase), the cyclone and associated exhaust gas ducts. Such overheating causes many operational deficiencies which include, but are not limited to, scaling, poor temperature control, increased energy requirements and poor product quality. Due to these operational problems, direct injection of liquid and gaseous fuels in high velocity CFB reactors is not typically practiced.
Several approaches have been utilized in order to avoid the problems associated with direct injection of liquid and gaseous fuels into CFB reactors. For example, reaction kinetics can be enhanced by using an oxygen enriched fluidized system. However, the utilization of an oxygen enriched fluidized system requires the use of an oxygen plant thus increasing the costs and complexity of such a system. The fluidizing gas is preheated using direct or indirect combustion. However, there are significant limitations on the amount of energy which can be inputted to the system by preheating the fluidizing gases.
Another attempt to solve the above-mentioned problems is to heat the CFB reactor by directly firing burners into the reactor. The burner can operate using either liquid or gaseous fuels. However, direct firing causes sintering, which is a loss of product quality for temperature sensitive feed particulate.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a new and improved circulating fluidized bed reactor system wherein combustion gases derived from liquid or gaseous fuels are introduced into the dense phase of the reactor without causing significant reduction of oxidation in the dense phase.
It is another object of the present invention to provide a new and improved circulating fluidized bed reactor system which utilizes burners external to the reactor which oxidizes liquid or gaseous fuels sufficiently so as to provide combustion gases having low levels of carbon monoxide and unoxidized hydrocarbons.
It is a further object of the present invention to provide a new and improved circulating fluidized bed reactor system which does not utilize an oxygen enriched fluidized system.
It is yet a further object of the present invention to provide a new and improved circulating fluidized bed reactor system wherein the combustion gases are quenched prior to being introduced into the transition or dense phase portion of the reactor so as to prevent sintering or overheating of the feed particulate.
It is another object of the present invention to provide a new and improved circulating fluidized bed reactor system wherein the quenching air utilized to quench the combustion gases may be preheated by processed particulate.
It is further object of the present invention to provide a new and improved circulating fluidized bed reactor system which can be constructed and operated for reasonable costs.